Question: How do I use my manometer to determine when its time to change the exhaust filters?
Answer: While different jurisdictions have different requirements when to change your exhaust filters, this will outline how to get a correct reading using a Dwyer Mark II manometer. First thing to do is understand the difference between FILTER pressure drop and TOTAL pressure drop.Filter pressure drop is the amount of resistance a filter has against the airstream and Total pressure drop is the amount of resistance a filter has against the airstream plus a number of variable conditions such as duct size, quantity and size of fan, size of plenum, quantity and quality of filters and spray chamber resistance. Lets take what we have learned and apply it to your booth, we just want to know what the pressure drop of your filter is, to do this we need to establish a baseline or "clean condition" for point of reference. We are not interested in knowing any readings from external sources not directly related to the actual filter.Replace your filters and with the booth OFF, adjust the manometer to read zero, making sure the low side is connected to the downstream side of the filter and the high side is connected to the upstream side of the filter. Check for obstructions in the tubing and fittings as this will affect your reading.Start your booth and record your "clean condition" reading, in this example we are using .50 W.C. (that’s a 1/2" negative pressure).Now add your local air pollution district filter change requirement, example shown is .25" W.C. (that’s 1/4" negative pressure). Combine the two and you get your maintenance interval. With a total of .75" W.C. filter pressure drop before you are legally required to change your filters.While your baseline "clean condition" numbers may be very low or very high depending on the variables, the filter pressure drop and maintenance interval remains a constant. Are you in a part of the country that doest have an air pollution board? Use the following numbers to monitor your filters condition. These numbers do NOT include the baseline. Fiberglass will perform up to .5" W.C. / Expanded paper will perform up to 1.0" W.C. / Polyester paint pockets will perform up to .75" W.C.